Kneaders or so-called closed mixers have been known for a long time and have a closed, usually elongated kneading chamber, in which are located two parallel oriented rotors which are rotated in opposite directions. As a function of the desired nature and quantity, different components of a mixture, particularly high molecular weight, viscoelastic materials, e.g. polymers or elastomers, are introduced through a filling shaft into the kneading chamber. The components of the mixture are exposed to shear forces in the kneading chamber by means of the rotors, so that they are transformed or kneaded to a very homogeneous, viscoelastic mass.
The processing or flow behaviour of high molecular weight materials such as polymer melts or elastomers is determined by a combination of their viscous and elastic characteristics, an important influence being exerted inter alia by the nature and fractions of the components of the mixture and the quality of the distribution level of all the components. The components can be polymers having different molecular structures or can comprise pulverulent fillers with different particle sizes in a differing particle size distribution.
If e.g. high molecular weight and consequently also highly elastic, fractions of a first component are introduced into a low molecular weight, low viscosity, second component and the components are mixed in a turbulent manner in a kneader with two rotors rotated in opposite directions, theoretically following an adequately long mixing time a constant, relatively high degree of mixing is at least asymptotically reached. However, it has been found that this is not the case if the viscoelastic, first component, due to the action of the rotors, is only briefly expanded and then reverts to its original particle form. In this case it must be ensured that the component fractions during kneading are reduced to the smallest possible size, and the thus formed parts are distributed as homogeneously as possible in the mass. The extent of this homogenization process depends on the geometrical conditions of the kneader used, the chosen processing conditions, e.g. the mass temperature, rotor speed and degree of filling of the kneading chamber, and in particular on the viscous and elastic characteristics or parameters of all the components.
A kneading or thorough mixing by the introduction of relatively high shear forces, both onto the high molecular weight, first component and onto the low molecular weight, second component, is intended to lead to a uniform mastication of both molecular structures, and inter alia the molecular chain length of both polymer components, and consequently the viscosity of the total mass are reduced, and the thorough mixing of all the components is facilitated. However, mastication is often more effective in the case of the low molecular weight, second component than of the high molecular weight, first component, whose fractions "float" in the second component and consequently are not sufficiently strongly subject to the action of the shear forces to bring about the desired breaking up of the molecular chains. Therefore the pronounced elastic characteristics of the first component have a marked influence on the viscoelastic characteristics of the overall mixture.
When kneading or thoroughly mixing different types of components, for the aforementioned reasons it is impossible or difficult to forecast how the viscous and elastic properties of the mixture will change during the kneading process. It is therefore impossible to make a reliable forecast as to the extent by which the processing characteristics of the mixture are changed by the kneading process or what use properties the mass will ultimately have. It is consequently of interest to be able to follow both the viscous and elastic characteristics of a mixture during the kneading process as a function of the processing conditions and the mixing time.
In the case of kneaders it is conventional practice (U.S. Pat. No. 3,447,201, EP 0 392 787 A1) to determine the change to the viscous characteristics, e.g. of a polymer mixture, in that on presetting a constant rotor speed determination takes place of the time pattern of the resistance opposed to the rotors by the mass in the kneading chamber, in that the necessary drive motor power is measured. Particularly in the case of smaller laboratory kneaders it is possible to determine the time change of the torque acting on the drive spindles at a constant rotor speed. In such a case the measured torque is considered to be a viscosity parameter of the mass. Kneading in a kneader presupposes that for obtaining an intense thorough mixing the mass is exposed to a turbulent movement or flow in the kneading chamber. As there is no laminar state in the kneading chamber, as is required for absolute viscosity measurements in rheometers, the viscosity parameter determined from the torque of the drive mechanism only has a relative character and in addition the kneader geometry and other kneading parameters have a significant influence on the magnitude of said parameter or characteristic value. Different types of kneaders and varying test parameters consequently always give different viscosity-proportional torque values for identical samples. For example within the framework of quality control, these relative viscosity parameters acquire significance from comparison with the evaluation of the use characteristics of the end product. However, tests with conventional kneaders are unable to provide information on the elastic characteristics of the investigated viscoelastic masses, although specifically in the case of high molecular weight polymers and elastomers the elastic characteristics are often much more important than the viscous characteristics with regards to processability.
It is known from rheology to determine the elastic characteristics of viscoelastic masses in so-called rotary rheometers, where a sample of the mass is exposed to precisely defined shear conditions. For this purpose a sample of the mass is placed in the gap between a rotor plate and a stator plate. If the rotor plate is moved with a small angular amplitude on applying a sinusoidal, oscillating torque, it is possible to establish whether the sample oscillates in equiphase manner with the preset torque, or whether the deformation of the sample with respect to the preset torque takes place with a phase shift angle .delta.. A substance is classified as purely elastic if the shift angle .delta. is 0.degree.. The substance is classified as purely viscous if the angle .delta. is 90.degree.. For an angle .delta. between 0 and 90.degree., the substance is called viscoelastic.
In the case of such dynamic measurements, apart from the phase shift angle .delta., determination also takes place of the complex modulus G*, which represents the total resistance of the test mass against the forced deformation. On the basis of the quantities .delta. and G* by applying known mathematical relationships the storage modulus G', which is proportional to the elasticity, and the loss modulus G", which is proportional to the viscosity, can be determined. With a constant oscillation frequency, the time changes of these dynamic measured quantities can be determined as a consequence of e.g. different mixing intensities. Alternatively the viscoelastic behaviour of the test mass can be determined over oscillation frequencies modified in a predetermined manner. With a correct choice of the test parameters, e.g. the geometrical conditions in the measuring chamber, the dynamic moduli G' and G" have the character of absolute quantities, so that they are not dependent on specific rheometers or specific measuring devices. However, it is important for these measurements that the mass is only exposed to very low shear forces and correspondingly small reversible deformations, and that the measurements take place when the mass is in the rest state. During the operation of the kneader, with turbulent movement prevailing in the kneading chamber, it is not possible to determine with sufficient accuracy the dynamic parameters of a mass.
The problem of the invention is to provide a kneader of the aforementioned type, in which elastic and/or viscous parameters of the viscoelastic mass to be kneaded can be determined with an adequate precision, and also followed over time. In addition, a corresponding method for the rheometric analysis of a viscoelastic mass is to be provided.